When centrifuging blood to achieve phase separation, a stoppered test-tube is commonly used in which the phases separate in response to the centrifugal force, the heavier cells going to the bottom of the tube and the lighter serum or plasma towards the stoppered end. Since 1920, it has been known that the phase separation occurs more rapidly if the axis of the test tube is inclined at an angle, rather than parallel, to the direction of centrifugal force (which extends radially from the rotor). Boycott, "Sedimentation of blood corpuscles," Vol. 104 of Nature, p. 532.
Attempts have been made to make use of such more-rapid phase separation, but largely they have relied upon spinning techniques that require specialized separation tubes, such as those shown in U.S. Pat. No. 5,030,341. These require that spinning be about the axis of the tube, thus of course preventing the use of conventional plain tubes.
Furthermore, it has become conventional to use a gel separator in the tube which locates itself between the two phases during centrifuging, to seal them off so that separation is maintained without having to immediately pour off (decant) the supernatant serum. For example, such tubes can be obtained under the trademark "Vacutainer Plus" from Becton-Dickinson. However, those tubes include instructions that state the gel seal is maintained only if the rotor uses a "horizontal head". That is, the gel seal integrity can be relied upon only if the tube is centrifuged so that its long axis is parallel to (aligned with) the direction of centrifugal force. The effect, apparently, is that inclining the long axis at an angle to that centrifuge direction stretches the gel cross-section diameter and reduces its thickness, all of which hinder the formation of an effective seal.
Hence, there are two contradictory effects that, prior to this invention, have not been reconciled: The need to centrifuge a tube with a gel barrier so that the long axis is not aligned with the centrifugal force directions, to make use of the "Boycott" effect noted above for more rapid phase separation; and, the need to centrifuge the tube with the long axis aligned with the force direction, to ensure the gel will seal across the phase boundary. Thus, there has been a need ever since the gel-tube was introduced, to find a way to reconcile these competing interests. (To date, the more traditional approach has been to abandon the Boycott effect in favor of producing a reliable gel seal.)